Non-aqueous secondary batteries (hereinafter, also referred to simply as “secondary batteries”) such as lithium ion secondary batteries have characteristics such as compact size, light weight, high energy-density, and the ability to be repeatedly charged and discharged, and are used in a wide variety of applications. A non-aqueous secondary battery normally includes battery components such as a positive electrode, a negative electrode, and a separator that separates the positive electrode and the negative electrode to prevent short-circuiting between the positive electrode and the negative electrode.
In secondary batteries, battery components including functional layers that impart desired performance (e.g. heat resistance or strength) to the battery components are used. Specific examples of such battery components used include a separator that is obtained by forming a functional layer on a separator substrate and an electrode that is obtained by forming a functional layer on an electrode substrate formed by providing an electrode mixed material layer on a current collector. As a functional layer capable of improving the heat resistance, strength, or the like of a battery component, a functional layer composed of a porous membrane layer formed by binding non-conductive particles by a binder is used. For example, this functional layer is formed by applying, to a surface of a substrate (such as a separator substrate or an electrode substrate), a composition for a functional layer containing non-conductive particles, any of various polymers capable of functioning as a binder, and a dispersion medium and drying the applied composition for a functional layer.
In recent years, there has been a large amount of activity directed toward improving compositions for a non-aqueous secondary battery functional layer that are used in the formation of functional layers, with an objective of providing secondary batteries having even higher performance (for example, see WO 2015/145967 A1 (PTL 1)).
Specifically, PTL 1 proposes a technique of containing, in a composition for a porous membrane which is a composition for a non-aqueous functional layer, a particulate polymer that is a random copolymer including at least 35 mass % of a (meth)acrylic acid alkyl ester monomer unit and at least 30 mass % and not greater than 65 mass % of an aromatic monovinyl monomer unit and has a degree of swelling of more than 1.0 time and 2.0 times or less in a non-aqueous electrolyte solution. A composition for a functional layer containing a particulate polymer having such a composition and properties improves the durability of a porous membrane forming a functional layer and has enhanced stability under high shear.